Currently, a photodiode for optical communication having an InGaAs light receiving layer formed on an InP substrate is used. For optical communication, near infrared of 1.3 μm band and 1.55 μm band are often used for transmission/reception. Therefore, mixed crystal ratio (Ga=0.47, In=0.53) of InGaAs is determined from matching condition with an InP substrate, and photosensitivity is limited to light of up to 1.6 μm. Though a photodiode can sense and receive light having the energy equal to or higher than the band gap Eg, it is not sensitive to light of which energy is not higher than the band gap Eg.
Infrared light having long wavelength of 1.6 μm to 3 μm can be received only by a semiconductor material having a very small band gap Eg. It is difficult to find a material having such a narrow band gap. Even if a material satisfies the condition of narrow band gap, the material might not grow or not well match the substrate. Even if successfully grown, a light emitting device may not emit light, and even if successfully grown, a light receiving device may not be practically used, as it may have too large dark current or unsatisfactory sensitivity.
Japanese Patent Laying-Open No. 09-219563 (Patent Document 1) proposes a light emitting element and a light receiving element that have quaternary mixed crystal of GaInNAs as a photoactive layer and a light absorbing layer. This proposes a light emitting device and a light receiving device that emit and receive mid infrared light of 1.7 μm to 5 μm. By appropriately selecting the mixed crystal ratio, GaInNAs band gap of 0.73 eV or smaller can be attained. Further, it is asserted that GaInNAs allows formation of high-quality crystal with very few crystal defects that can attain lattice matching with an InP substrate.
Japanese Patent Laying-Open No. 2003-282927 (Patent Document 2) aims to fabricate a photodiode for optical communication having small dark current and sensitivity (of about 1.63 μm) closer to L-band edge (1700 nm). A plurality of layers of In0.53Ga0.47As having a lattice constant smaller than InP and In0.55Ga0.45As having a lattice constant larger than InP are stacked and lattice matching with InP is attained. By alternately stacking thin films having lattice constant smaller than the lattice constant of the substrate and thin films having lattice constant larger than that of the substrate, quasi lattice matching is established. The resulting device, however, cannot sense and receive light of 1.7 μm or longer. An object of the present invention is to provide a light receiving device having a narrow band gap that can sense and receive light having the long wavelength of 1.7 μm to 3 μm.
T. Murakami et al., “InxGa1-xAs/InAsyP1-y detector for near infrared (1-2.6 μm)”, Conference Proceedings of Indium Phosphide and Related Materials (Non-Patent Document 1) proposes a mid-infrared photodiode having an InGaAs light receiving layer. To satisfy the condition of attaining lattice matching with InP, in conventional photodiodes, the ratio was always In0.53Ga0.47As. The conventional example having 53% In has wide band gap and, therefore, mid infrared light having the wavelength of 1.6 μm or longer cannot be received. In Non-Patent Document 1, among InGaAs mixed crystals, In0.82Ga0.18As is used, in which the ratio of Ga is decreased and the ratio of In is increased to make the band gap narrower, as the light receiving layer.
It is asserted that as the ratio of InP increases, the band gap becomes narrower and as the band gap becomes narrower, sensitivity to infrared light having the wavelength up to 2.6 μm can be attained. The device, however, has a problem that significant lattice mismatch between the InGaAs light receiving layer and the InP substrate causes large number of lattice defects and eventually leads to high dark current. Therefore, in the device of Non-Patent Document 1, graded layers of InAsyP1-y having the value y varied little by litter among 12 to 20 layers are interposed between the InP substrate and the InGaAs light receiving layer. This is said to be effective to decrease dark current. The dark current, however, is about 20 μA to about 30 μA, which is still too high.
J. W. Matthews and A. E. Blakeslee, “Defects in Epitaxial Multilayers”, J. Cryst. Growth Vol. 27 (1974), pp. 118-125 (Non-Patent Document 2) discloses calculation of a critical thickness at which occurrence of misfit dislocation can be prevented when a GaAsP mixed crystal thin film is epitaxially grown on a GaAs substrate. Though the types of substrate and thin film are much different from those of the present invention, it is important as the critical thickness is clearly given. This will be discussed later.    Patent Document 1: Japanese Patent Laying-Open No. 09-219563 “Semiconductor Light Element, and Application System Using It”    Patent Document 2: Japanese Patent Laying-Open No. 2003-282927 “Photodiode”    Non-Patent Document 1: T. Murakami et al., “InxGa1-xAs/InAsyP1-y detector for near infrared (1-2.6 μm)”, Conference Proceedings of Indium Phosphide and Related Materials    Non-Patent Document 2: J. W. Matthews and A. E. Blakeslee, “Defects in Epitaxial Multilayers”, J. Cryst. Growth Vol. 27 (1974), pp. 118-125    Non-Patent Document 3: W. A. Jesser and J. W. Matthews, “Evidence for Pseudomorphic Growth of Iron on Copper”, Phil. Mag. Vol. 15 (1967) pp. 1097